Many motor vehicle tires have a circumferential tread provided with a plurality of circumferential grooves that define ribs therebetween. Typically, generally lateral slots can be provided in the ribs to form a plurality of shaped blocks. These shaped blocks can be distributed along the tread according to a specific pattern. Sipes, which are generally narrow slits cut into the tread, can be provided in the shaped blocks in a specific pattern.
Within the tread pattern in which the circumferential grooves are provided, the shaped blocks are typically distributed in several side by side rows. A proper distribution of the shaped blocks gives the tire the desired characteristics of directional control stability and road gripping in relation to the sidewise thrusts directed parallel to the wheel axis. The lateral slots, in turn, give the tire the desired road traction, that is the capability of efficiently transmitting tangential thrusts parallel to the running direction during speeding up and slowing down of the vehicle. In principle, the traction of the tire tends to become greater as the number of the lateral slots arranged in the tread increases and the orientation of the slots themselves approach an orientation that is perpendicular to the running direction of the tire. Additionally, the circumferential grooves and lateral slots cooperate in performing an efficient draining action of water from the tread pattern area of the tire during running on a wet road bed. Finally, the sipes, if present in the shaped blocks, can improve wet traction of the tire as well as snow and ice traction.
The presence of the longitudinal grooves and lateral slots, however, causes the well known effect of noise produced by a rolling tire. One type of noise, known as organ pipe noise, is a consequence of the circumferential grooves that become active in the contact patch of the tire. A pressurized air column is formed in the circumferential grooves between the ground contact portion of the tire and the road surface, and this produces sound as a consequence of the resonance of the air column. The wavelength of the sound is substantially constant and is independent of the speed of the tire.
Another type of tire noise, known as pumping noise, can be generated by a high speed flow of air in the lateral slots that can also activate noise from the circumferential grooves. Yet another type of noise, known as pitch noise, can be generated by the tread pattern elements as a consequence of their circumferential pitch arrangement. Finally, if sipes are present in the shaped blocks, sizzle noise can be generated by the rubbing of the parallel walls of the sipes when shaped blocks deform as they impact the road bed.